A pressure vessel for storing a fluid medium (such as hydrogen) typically includes a pressure chamber which contains the pressurized fluid medium that is moved in a controlled manner in or out of the vessel. In certain designs, the pressure vessel includes an inner plastic liner with two metal mouthpieces (bosses) arranged on opposite ends, and a filament wound outer shell to support the inner vessel liner.
Typically, the vessel liner is manufactured using a conventional injection molding, blow molding, rotational molding process, or thermoforming process. For example, the following patents describe a vessel liner (WO 1999/039896 Dulisse, JMK; WO 2007/079971 Müller, Xperion; DE 19526154 Lange, Mannesmann etc.; and WO 1999/013263 Jensen, Raufoss), each of which is incorporated herein by reference in its entirety.
To permit controlled movement of fluids in or out of the pressure chamber, the vessel is typically configured with a pressure chamber orifice and a boss is fitted in the orifice. The boss is threaded or otherwise shaped for connection to nozzles, valves, gauges, tubes, and similar fixtures which direct and control fluid flow. Accordingly, the boss is formed of a metal or another conventional material having desired properties.
The boss typically includes a cylindrical neck with a longitudinal passage that provides fluid communication between the pressure chamber and the environment outside the vessel. A longitudinal axis is defined within the neck as substantially parallel to a direction the passage between open ends thereof. In certain designs, a flange is secured to one end of the neck. The flange, which is larger than the pressure chamber orifice, is secured to the liner of the pressure vessel to militate against relative movement between the boss and the liner.
First, it is desirable to militate against a translation of the boss, in respect of the liner. That is, the boss should not be permitted to move along the central longitudinal axis of the neck such that it falls completely inside the pressure chamber. Nor should the boss be allowed to move in the opposite direction and separate itself from the vessel.
Second, it is desirable to militate against a rotation of the boss, in respect of the liner. That is, the boss should not be permitted to rotate with respect to the liner about the central axis of the neck or about the pressure chamber orifice. Thus, when the vessel is held and an attempt is made to thread a fixture into the threaded boss, the boss should not thwart the attempt by rotating with respect to the liner.
The problem therefore arises of how to securely attach the boss to a non-metallic liner to prevent translation and rotation of the boss while providing a substantially fluid-tight seal between the boss and the vessel liner. One approach simply relies on the internal pressure in the vessel to attach the boss and liner. Although this pressure may suffice to create a seal between the boss and the non-metallic liner in some vessels, and may also prevent boss translation, it generally does not prevent boss rotation.
Another approach positions the boss between two lips of a partially bifurcated liner. Such a liner includes two lip layers disposed about the circumference of the pressure chamber orifice. The boss flange is positioned in an annular recess between the lip layers and is encapsulated by the liner lips. No adhesive is used between the boss and the liner lips. Although this approach limits boss translation, it fails to substantially restrict boss rotation.
A different approach to securing the boss to the liner includes placing a layer of adhesive between the boss and the liner. The adhesive bonds the boss and the liner together, at least initially. Unfortunately, some adhesives deteriorate over time. Thus, the adhesive may not last the entire service life of the vessel, particularly if pressure and thermal stresses or chemical contaminants are introduced into the vessel.
It is also difficult to bond metal bosses to certain thermoplastic materials such as nylons and polyolefins, because petroleum-based thermoplastic materials inherently have a so-called “lubricated” surface. The use of adhesives to bond metal to such materials typically involves elaborate surface preparation and treatment steps to ensure an adequate bond. The repeatability of adequate bonding also becomes operator sensitive, and hence, problematic in large volume manufacturing.
It would be desirable to develop a boss for use with a pressure vessel and a method for forming an inner liner of the pressure vessel including the boss, wherein the boss is securely coupled to the liner to form a substantially fluid-tight seal between the boss and the vessel liner.